1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same, and to a thin-film magnetic head, a head assembly and a magnetic disk drive each including the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include GMR (giant magnetoresistive) elements utilizing a giant magnetoresistive effect, and TMR (tunneling magnetoresistive) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the spacer layer. The free layer is a ferromagnetic layer having a magnetization that changes its direction in response to a signal magnetic field. The pinned layer is a ferromagnetic layer having a magnetization in a fixed direction. The antiferromagnetic layer is a layer that fixes the direction of the magnetization of the pinned layer by means of exchange coupling with the pinned layer. The spacer layer is a nonmagnetic conductive layer in spin-valve GMR elements, or is a tunnel barrier layer in TMR elements.
Read heads incorporating GMR elements include those having a CIP (current-in-plane) structure in which a current used for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and those having a CPP (current-perpendicular-to-plane) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element.
Read heads each have a pair of shields sandwiching the MR element. The distance between the two shields is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
As an MR element capable of reducing the read gap length, there has been proposed an MR element including two ferromagnetic layers each functioning as a free layer, and a spacer layer disposed between the two ferromagnetic layers (such an MR element is hereinafter referred to as an MR element of three-layer structure), as disclosed in U.S. Pat. No. 7,035,062 B1, for example. In the MR element of three-layer structure, the two ferromagnetic layers have magnetizations that are in opposite directions when no external magnetic field is applied to the layers and that change directions in response to an external magnetic field.
In a read head incorporating the MR element of three-layer structure, a bias magnetic field is applied to the two ferromagnetic layers. The bias magnetic field changes the directions of the magnetizations of the two ferromagnetic layers so that the directions of their magnetizations each form an angle of approximately 45 degrees with respect to the direction of track width. As a result, a relative angle of approximately 90 degrees is formed between the directions of the magnetizations of the two ferromagnetic layers. When a signal magnetic field from the recording medium is applied to the read head, the relative angle between the directions of the magnetizations of the two ferromagnetic layers changes, and as a result, the resistance of the MR element changes. With this read head, it is possible to detect the signal magnetic field by detecting the resistance of the MR element.
Read heads incorporating MR elements of three-layer structure allow a greater reduction in read gap length, compared with read heads incorporating conventional GMR elements. For example, in a CPP-structure read head incorporating a conventional GMR element, the read gap length can be reduced to approximately 30 nm at the smallest. In contrast, in a CPP-structure read head incorporating an MR element of three-layer structure, the read gap length can be reduced to approximately 20 nm or smaller.
One of methods for making the magnetizations of the two ferromagnetic layers of an MR element of three-layer structure be in opposite directions when no external magnetic field is applied thereto is to antiferromagnetically couple the two ferromagnetic layers to each other through the spacer layer by means of the RKKY interaction. The technique of antiferromagnetically coupling the two ferromagnetic layers to each other by means of the RKKY interaction is utilized for a so-called synthetic pinned layer of a spin-valve GMR element.
In the case of antiferromagnetically coupling the two ferromagnetic layers to each other through the spacer layer by means of the RKKY interaction in the MR element of three-layer structure, it is required to increase the strength of the antiferromagnetic coupling between the two ferromagnetic layers so as to allow the MR element to operate with stability. The strength of the antiferromagnetic coupling depends on factors such as the material used for the two ferromagnetic layers, the thickness of the spacer layer, the state of the interface between the spacer layer and each of the ferromagnetic layers.
When an MR element of three-layer structure is used as a CPP-structure MR element, it is sometimes preferred to use such a material that the spin-dependent scattering of electrons increases in a ferromagnetic layer as the material to form the two ferromagnetic layers, in order to increase the magnetoresistance change ratio (hereinafter referred to as the MR ratio), that is, the ratio of magnetoresistance change with respect to the resistance of the MR element. However, such a material may often be one that would reduce the strength of the antiferromagnetic coupling between the two ferromagnetic layers. Therefore, in order to make the strength of the antiferromagnetic coupling between the two ferromagnetic layers sufficiently high even when such a material is used, it is desirable to achieve a maximum strength of the antiferromagnetic coupling between the two ferromagnetic layers by controlling the conditions other than the material of the two ferromagnetic layers, such as the state of the interface between the spacer layer and each of the ferromagnetic layers.
As thus described, for the MR elements of three-layer structure, there is a need for controlling the state of the interface between the spacer layer and each of the ferromagnetic layers from the viewpoint of increasing the strength of the antiferromagnetic coupling between the two ferromagnetic layers. Here, with regard to the state of the interface between the spacer layer and each of the ferromagnetic layers, a factor that would reduce the strength of the antiferromagnetic coupling between the two ferromagnetic layers is crystal lattice mismatch at the interface between one of the ferromagnetic layers formed before the spacer layer and the spacer layer formed thereon.
In a typical spin-valve GMR element, the two ferromagnetic layers sandwiching the spacer layer are not antiferromagnetically coupled to each other, and therefore it is not necessary to control the state of the interface between the spacer layer and each of the ferromagnetic layers from the above-mentioned viewpoint. For example, JP 2002-092826A discloses a spin-valve GMR element of the CPP structure having a configuration in which at least one of the pinned layer and the free layer sandwiching the spacer layer is made up of ferromagnetic layers and nonmagnetic layers that are alternately stacked. In this GMR element, the pinned layer and the free layer sandwiching the spacer layer are not antiferromagnetically coupled to each other, and therefore, as a matter of course, it is not necessary to control the state of the interface between the spacer layer and each of the ferromagnetic layers so as to increase the strength of antiferromagnetic coupling between the two ferromagnetic layers.